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INTRODUCTION.                                               17
and one base.   Such a complex slag, all other things being equal, will be more fluid in the furnace than a simple slag.
(7)   In all furnaces, whether acid or basic, there is more or less of an automatic regulation.    In the acid furnace the percentage of silica will be constant, for if there is not enough silicon in the charge to supply the necessary silica, the slag will eat away the bottom until it is satisfied.   The total content of the oxides of iron and manganese will be constant, for if there is no ore added, the iron of the bath will be oxidized.    If ore is added, the silicon and carbon of the bath unite with the oxygen of the ore and the iron goes into the bath.    Thus the slag takes care of itself on an acid hearth.
(8)   In the basic furnace the slag takes care of itself to some extent, but the cutting away of the hearth must not be allowed, and if phosphorus is to be eliminated, a sufficient quantity of lime must be added.    Given the right amount of lime, there is then a considerable self-adjustment of the slag by the oxidation of the iron of the bath or by the reduction of the iron from the slag.    If much lime be added, it will tend to drive the iron back into the bath, although it can never do it completely, while if little lime IK added, there will be a greater proportion of iron in the slag.
(9)   It is necessary that the slag shall be so basic that it will not attack the bottom.   If it is so, it is basic enough to hold all the phosphorus that will be present if the stock contained only a moderate amount—say not over one-half of one per cent.    If the stock contained far in excess of this, as often happens, special attention must be paid that phosphorus does not pass back into the steel when a high temperature is combined with violent agitation and perhaps a reducing action, these conditions being often present when the heat is tapped.
Every engineer knows that steel is not homogeneous. Manufacturers have always known it, but they have usually said very little about it. It is a much safer plan to state the facts and let proper allowance be made in the proper place. The tendency among structural engineers is continually toward heavier work. The size of beams and angles and girders is greater now than it was some years ago, and the percentage of the heavy flections is greater. These heavy pieces necessarily mean heavy ingots ia